University teacher's crucial creature (6)
I believe the answer is:
donkey
'creature' is the definition.
(type of animal)
'university teacher's crucial' is the wordplay.
'university teacher' becomes 'don' (Oxbridge term for an academic).
'crucial' becomes 'key' (I've seen this in another clue).
'don'+'key'='DONKEY'
(Other definitions for donkey that I've seen before include "Braying animal" , "Mary's transport" , "Domesticated ass" , "Nativity play animal?" , "Braying quadruped" .)